Semiconductor light-emitting devices such as light emitting diodes are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III–V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Light emitting devices based on the III-nitride materials system provide for high brightness, solid-state light sources in the UV-to-yellow spectral regions. Typically, III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Some of these substrates are insulating or poorly conducting. Devices fabricated from semiconductor crystals grown on such substrates must have both the positive and the negative polarity electrical contacts to the epitaxially-grown semiconductor on the same side of the device. In contrast, semiconductor devices grown on conducting substrates can be fabricated such that one electrical contact is formed on the epitaxially grown material and the other electrical contact is formed on the substrate. However, devices fabricated on conducting substrates may also be designed to have both contacts on the same side of the device on which the epitaxial material is grown in a flip-chip geometry so as to improve light extraction from LED chip, to improve the current-carrying capacity of the chip, or to improve the heat-sinking of the LED die. Two types of light emitting devices have the contacts formed on the same side of the device. In the first, called a flip chip, light is extracted through the substrate. In the second, light is typically extracted through transparent or semi-transparent contacts formed on the epitaxial layers.
The use of a substrate with a low index of refraction, such as sapphire, may lead to poor optical extraction efficiency in a flip chip device due to the large different in index of refraction at the interface between the semiconductor layers and the substrate. FIG. 1 illustrates the interface between a GaN layer 11 and a sapphire substrate 12. When light ray 10a is incident on the interface, a portion 10c is transmitted into the sapphire and a portion 10b is reflected back into GaN layer 11. In the regime in which classical optics apply, the angle of transmission is governed by Snell's law: nsapphiresin T=nGaNsin I, where nsapphire is the refractive index of sapphire (1.8), nGaN is the refractive index of GaN (2.4), T is the angle of transmission, and I is the angle of incidence. When light is incident on the interface at an angle larger than a critical incidence angle, all of the incident light is reflected back into the GaN. For light propagating through GaN and incident on sapphire, the critical incidence angle is about 50°. Reflected light may make many passes through the device before it is extracted, if it is extracted at all. These many passes result in significant attenuation of the light due to optical losses at contacts, free carrier absorption, and interband absorption within any of the III-nitride device layers.
One way to reduce the amount of light reflected back into the GaN layer in a flip chip device is to include a scattering structure at the interface between the GaN and the substrate. The scattering structure interrupts the smooth interface such that a smaller amount of light strikes the interface at an angle larger than the critical angle, resulting in a larger amount of light entering the substrate. In U.S. Pat. No. 6,091,085, titled “GaN LEDs With Improved Output Coupling Efficiency,” a sapphire substrate is roughed prior to the formation of GaN device layers. The roughened surface is a scattering structure that increases the amount of light transmitted into the substrate. The substrate may be roughed mechanically, such as by scratching the surface with grinding grit, or by photolithographically patterning the substrate. Using a roughened substrate surface as a scattering structure has several disadvantages. Mechanical roughening creates a non-reproducible substrate surface. Since the substrate surface can impact the quality of the III-nitride device layers grown over the substrate, the use of non-reproducible substrates can cause unacceptable variations in brightness and efficiency between devices. In addition, if the substrate surface is too rough, III-nitride device layers of sufficient quality for light emitting devices may not grow on the substrate. Photolithographic patterning and etching of sapphire is costly, and can also result in a substrate that is inappropriate for growth of III-nitride device layers.